The present invention relates to a method of sensing access positions of the arm of a transfer mechanism in a semiconductor equipment system.
In recent semiconductor device processes, sheet semiconductor equipment systems of the multi-chamber type with a systematically clustered tool structure have been used. In this type of system, various process chambers for deposition processes, such as sputtering, or CVD, and etching processes, such as RIE, and load/unload chambers for externally loading and unloading an object to be processed are arranged around a transfer chamber with a transfer mechanism. These chambers are connected to the transfer chamber through gates.
In the semiconductor equipment system, a cassette that holds more than one object to be processed, for example, a silicon wafer (hereinafter, referred to as a wafer), is first placed on a stage that can be moved in the load/unload chamber.
Then, a wafer is taken out of the cassette by the transfer arm of a transfer mechanism provided in the transfer chamber and conveyed into a specified process chamber via the transfer chamber.
The wafer conveyed into the process chamber is subjected to specific processes including an etching process, a sputtering process, and a CVD process. After the specific processes, the wafer is conveyed through the transfer chamber and housed in the cassette again.
The cassette has slots in each of the side walls facing each other, each slot for one wafer, and can house wafers, for example 25 wafers, in parallel.
In general, when a wafer is loaded and unloaded into and from the cassette in the load/unload chamber through the transfer chamber, the access position of the transfer arm has to be adjusted beforehand so that each slot in the cassette can be accessed. Using, for example, an optical sensor provided in the load/unload chamber, the following four access positions of the transfer arm are found for each cassette with respect to a pick formed on the transfer side of the transfer arm:
(1) Access position when the bottom slot has no wafer PA1 (2) Access position when the bottom slot has a wafer PA1 (3) Access position when the top slot has no wafer PA1 (4) Access position when the top slot has a wafer
Information on these positions is stored in a specific memory device.
Then, a cassette stage on which the cassette has been placed in the load/unload chamber and the transfer arm operate on the basis of information on the access positions, thereby transferring the wafer in the desired state.
To determine the four access positions for each cassette with respect to the pick of the transfer arm, however, the operator must be skillful in the work because a method of sensing and setting the positions is very complex.
Moreover, in the method, the direction in which the light beam is projected, that is, the parallelism of the light beam, has to be adjusted for each slot in each cassette before the sensing. Therefore, it takes time to start to sense the access positions, which results in a decrease in the throughput.
Furthermore, when the parallelism of the light beam has collapsed at only one of the slots, this prevents the access position on the cassette from being sensed.
In addition, in the method of sensing the access position of the transfer arm, it is very difficult to sense cross slots. In cross slots, a wafer is housed obliquely in two slots. In the place where cross slots are present, there is a strong possibility that the advance of the transfer arm will damage the wafer.
Furthermore, after the cassette has been used for a long time, strains take place in the cassette, which causes the following problem: the position in which the wafer is placed is shifted from the proper position gradually. As a result, the wafer cannot be placed in the proper position, making it difficult to calculate the access position of the transfer arm. This may lead to an increase in the number of errors in the transfer of the wafer.
In a case where a wafer on which an orientation flat section has been formed is used, the weight balance of the wafer differs, depending on whether or not the orientation flat section is present. When the wafer is housed in such a manner that the orientation flat section aligns with the back of the cassette, the front of the wafer may hang down. As a result, the advance of the transfer arm can damage the wafer. The droop of the wafer's front can permit the transfer arm to bump against the wafer, which will possibly prevent the arm from advancing between the slots, making it impossible to transfer the wafer.